Sky & Telescope - October 2022 - 55

ALL IMAGES COURTESY OF THE AUTHOR
trail due to Earth's rotation. You'll also
find that the wide field of view increases
your compositional options, allowing
you to capture more of the night sky
(especially the Milky Way) while including
an interesting foreground that adds
visual appeal to your photos.
If you have access to a motorized
A Canon 300-mm, f/4 telephoto lens
provided the perfect framing for capturing the
beautiful Comet NEOWISE (C/2020 F3) as
it appeared in the evening sky in July, 2020.
This 30-second exposure was recorded with a
Canon EOS RP full-frame mirrorless camera set
to ISO 3200. An iOptron iEQ30 Pro motorized
equatorial mount carried the camera to prevent
trailed stars.
equatorial telescope mount, or to one of
the many battery-powered sky-trackers,
then you have more options. By counteracting
Earth's rotation, these mounts
allow for longer exposures and the use
of greater focal lengths that provide
extra detail, all without trailed stars.
You can also get away with slower apertures
(f/4, for example) while retaining
pinpoint star images despite the additional
exposure time required.
Either way, you can choose a basic
lens since features such as auto focus
and image stabilization aren't needed
for astrophotography. Quality zoom
lenses are versatile but rather expensive
compared to fixed-focal-length prime
lenses. These so-called prime lenses
(ones with a single focal length) are
likely to be less expensive and readily
available on the used market.
A Question of Focus
When checking out a lens in person,
pay particular attention to the quality
of its manual focus adjustment.
That's because the auto-focus systems
on most cameras struggle in dim light,
and you'll need to use manual focus
and " live view " (if your camera has that
feature) to achieve sharp stars. This is
one of those situations in which you
get what you pay for - it's simply easier
to focus a high-quality lens than a
low-end, budget option. Precise focusing
is another reason why a fast lens is
desirable. Beyond allowing for shorter
exposures, it provides a much brighter
live-view image.
Price is also often an indicator of
optical quality. When comparing two
similar spec'd lenses, the more expensive
model is likely to perform better
" wide open " , that is, at its maximum
(lowest f-number) setting. Such a lens
will render pinpoint-star images across
the entire frame. It's no use buying a
sk yand tele scope .o r g * OCTOBER 2022 55
fast lens if you have to stop it down to
a small aperture to produce acceptable
image sharpness. Each increase in f-stop
cuts the amount of light reaching your
camera's sensor in half - which is why,
ideally, you'll want to shoot with your
lens wide open.
Of course, no lens is perfect, but
some aberrations are more troublesome
than others. Image-editing software,
such as Adobe Lightroom or Photoshop,
can compensate for faults such as
vignetting, geometric distortion, and
even mild chromatic aberration. As long
as a lens doesn't exhibit severe coma
(in which stars are stretched out into
comet-like streaks in the corners of the
image) or field curvature (which prevents
the entire image from coming to focus
at once), it'll perform quite well.
Prime Time
For wide-field, fixed-tripod shots, there
are a number of high-quality options
available from a variety of different
companies. Of course, every camera
manufacturer makes (and promotes) its
own line of lenses, but these often come
at a premium price. If you own a fullframe
camera, a budget-friendly option
is the Rokinon/Samyang 14mm F2.8 ED
AS IF UMC lens, which is available for a
variety of common lens mounts. If price
is less of an issue, it's hard to beat the
excellent Sigma 14mm f/1.8 DG HSM
Art lens. For APS-C crop-sensor cameras,
check out the Rokinon/Samyang
10mm F2.8 ED AS NCS CS prime lens,
as well as the Tokina atx-i 11-20mm
f/2.8 CF zoom.
Although fish-eye lenses are somewhat
specialized, don't rule them out
if capturing super-wide vistas is your
main imaging goal. One of my favorites
is a discontinued Canon 15-mm model
that I picked up used for a good price.
Admittedly, it suffers from pretty severe
coma, but that's a defect I'm willing to
live with given its unique field of view
and fast (f/2.8) aperture. For cropsensor
cameras, an 8-mm fisheye lens
qMany quality lenses are available on the secondhand market for
reasonable prices. Shown here are a few the author has acquired
over the years (from left to right): a Canon EF 15mm f/2.8 fisheye,
Canon EF 10-22mm EF-S zoom, the EF 50mm f1.4 prime (mounted
on the Canon 60D camera), and the author's favorite telephoto, the
Canon EF 300mm f/4L IS USM lens.
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Sky & Telescope - October 2022

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